The Teenage Mutant Ninja Turtles, minus Michelangelo, are each killed in a way related to their personalities:
Raphael, who is always prone to get angry easily, goes to fight Karai and the Foot Clan alone to get back at them for ambushing Splinter, resulting in his death.
Leonardo, the most selfless and the one who puts the lives of others before his own, stays behind alongside Casey Jones to allow his allies and family to escape from Baxter Stockman.
Donatello, the most peaceful and technological member, accompanies Splinter for "peace talks" that end up with their deaths when he fails to use his smarts to get his father and himself to safety.
The first issue showing all the turtles with black bandanas during the Ronin’s hallucinations was obviously intended in-narrative as a means of concealing which of them survived to become the Ronin before April greeted him by name. However, another possible explanation for this could be found in the Ronin’s mental state; he doesn’t want to reflect on his past as it reminds him of everything he’s lost, so he avoids recalling the details that distinguished him from his brothers back in the day. Once he reunites with April and she acknowledges him as Michelangelo, having someone from his past there to help allows Mikey to acknowledge his brothers in more depth rather than act as though he’s just one of the now-broken group.
